1. Field of the Invention
The present invention relates to the general field of measuring and testing and, more specifically, to a suspension system used to suspend large space structures for the purpose of testing same in simulated weightless conditions.
2. Description of the Related Art
The proposed use of large space structure has created unique and challenging requirements. The extremely large size and light weight of the structures coupled with the near zero gravity, vacuum environment of space causes difficulty with the control and movement of such structures. To understand and document these difficulties, a significant number of ground-based tests on sample or model large space structures is required.
Ground vibration tests (GTV's) are necessary to characterize a structure's behavior under dynamic conditions such as slewing a large space antenna or a docking maneuver with a space station. GVT's provide data indicative of the structure's natural modes, frequencies, damping, and mode shapes. These data are critical in determining methods of moving and controlling large, light weight space structures..
One of the major difficulties with ground vibration testing of large space structures is suspending the structure to allow freedon of motion similar to that found in space. Although it is impossible to match the near zero gravity environment of space, it is desirable for suspension systems to support the structure in a manner that does not overly constrain its motion. The structural members themselves are not designed to take significant loads such as the weight loading which occurs in a ground test environment.
The extremely large size and relative frailty of the structures themselves, on even scale models of the structures, further adds to the problems associated with suspension systems. For example, elongated beam-type space structures typically have extremely low natural frequencies. The fundamental frequencies will often be below 0.5 Hz. To adequately uncouple the rigid body modes of the structure from the fundamental flexible modes, the rigid body modes should have frequencies substantially lower than the first fundamental mode. In the past, attempts have been made to suspend structures with long cables, in a pendulum fashion from a high ceiling. However, at the extremely low frequencies involved, the required cable lengths become prohibitively long.
Pendulum cable supports limit structural testing to one plane of motion at a time. This support technique ignores the coupling of structural modes in more than one plane and does not address the distortion of torsion modes of beam-like structures.
In additional to planar motions provided by pendulum suspension methods, a suspension system should also allow undistorted motion of the torsional degree of freedom of beam-type structures. Ideally, the pendulum cables should be attached at the node line of the beam first torsion mode. However, since this node line usually lies inside of the beam contour, it is likely to be inacessible for cable attachments.
Another alternative is to suspend the beam by means of very soft springs and pendulum cables attached to the structure on either side of the torsion node line. However, this approach is complicated by requirements regarding stress and stiffness of the cable/spring design.